1. Field of the Invention
The present invention relates to a physical quantity sensor that is capable of measuring a physical quantity, such as acceleration, applied from the outside by detecting a displacement amount of a movable portion that is formed by, for example, cutting from a silicon substrate.
2. Description of the Related Art
For example, a physical quantity sensor includes a movable portion that is made by etching a silicon substrate and that is supported so as to be movable in the height direction. In such a case, for example, in a structure described in Japanese Unexamined Patent Application Publication No. 2005-283393, a movable portion that is displaceable in the height direction is swingably supported via a deformable beam portion by a frame body that surrounds the movable portion. However, if a large physical quantity acts on the movable portion or if a physical quantity acts on the movable portion for a long time, the beam portion may become overloaded, so that damage or the like may be caused to the beam portion or the beam portion may not be able to recover to its original stationary state even when the action of the physical quantity disappears. Accordingly, the sensor sensitivity tends to decrease, and the sensor's life could not be effectively prolonged. In the structure described in Japanese Unexamined Patent Application Publication No. 2005-283393, a weight stopper is provided. However, the structure described Japanese Unexamined Patent Application Publication No. 2005-283393 is a complex structure in which a step is formed on a surface of the weight in a height direction and the weight stopper faces a recessed portion of the weight in the height direction. It is very difficult to form this structure by using MEMS technology.
With the existing structure described above, movement of the movable portion is unstable and fluctuating and deformation of the beam portion in the height direction is hindered, so that it was difficult for the movable portion to perform translational motion in the height direction in a stable manner and it is not possible to obtain precise sensor sensitivity.
The sensor may stop functioning if the movable portion, which has been displaced in the height direction, surface-contacts a facing surface that faces the movable portion and the movable portion sticks to the facing surface. To prevent this, the sensor has a sticking prevention structure for preventing sticking of the movable portion. The structure includes protrusions or the like disposed on the movable portion or on a facing surface that faces the movable portion.
However, it was difficult to precisely form a sticking prevention structure composed of, for example, protrusions, on the movable portion or on the facing surface that faces the movable portion.
Even with the configurations described in Japanese Unexamined Patent Application Publication Nos. 2005-283393 and 2008-197113, the stopper and the weight tend to surface-contact each other, and sticking tends to occur. Japanese Unexamined Patent Application Publication No. 9-127151 describes a configuration including stopper protrusions, which has been generally used.